Field of the Invention
The present invention generally relates to magnetic read heads formed by multilayer magneto-resistance elements which utilize a magneto-resistance effect and more particularly, to an arrangement for simplifying the structure of such multilayer magneto-resistance elements.
In rotating magnetic heads, such as those still commonly used in particular in video recorders to constitute read/write heads, the speed of rotation of the magnetic read heads is high and it is combined with the movement of the magnetic tape. As a result of the high relative speed between the tape and head, it is possible to extract a sufficient signal with a purely inductive read head. In such an inductive read head, the magnetic field coming from a track of a magnetic tape to be read is channeled by a magnetic circuit and produces a current in a solenoid joined to the magnetic circuit.
However, such rotating magnetic head arrangements suffer certain drawbacks, in particular in regard to the weight and the bulkiness of the mechanical parts which rotate at a relatively high speed and also relative to the reliability of operation.
An alternative to such rotating head arrangements consists in recording and in re-reading the tracks of the magnetic tape with devices which are stationary relative to the magnetic tape, for example, with a single device comprising one or more magnetic heads which operate either by writing, i.e., by recording, or by reading, or else with two devices, one of which is for writing and other for reading.
However, the movement of the magnetic tape is slow and if the magnetic heads are stationary relative to the tape, the variations of the magnetic field occur at relatively low speed (typically several centimeters per second), and it is impossible under these conditions to extract a sufficient signal using a purely inductive head.
As an alternative, magnetic read heads which use the magneto-resistance effect can be stationary in front of the magnetic tape, while delivering a sufficient useful signal.
In fact, sensors employing the magneto-resistance effect are currently used in various magnetic track reading devices. These sensors are placed in the vicinity of a mobile magnetic track, and their variations of electrical resistance are detected in response to the variations of the magnetic field recorded on the magnetic track of the tape.
Of the alloys commonly used to constitute a magneto-resistance element or sensor, in particular ferromagnetic alloys, such as, for example, permalloy (Fe.sub.80 Ni.sub.20), can be cited.
It should be noted that magneto-resistance elements can be separated into two types: 1) the mono-layer type in which the resistivity depends only on the relative orientation of the magnetization and the current in the material, and 2) the multilayer type in which the magneto-resistance effect is linked to a diffusion of the carriers dependent on their spin state.
The multilayer structure typically comprises a stack of at least two magnetic layers separated by a nonmagnetic layer, the two magnetic layers having an anti-ferromagnetic type coupling. One of the magnetic layers, hereinafter referred to as the "sensitive magnetic layer", has a coercive field which is considerably weaker than that of the other magnetic layer. The other magnetic layer which has the stronger coercive field is hereinafter referred to as the "stable magnetic layer". The sensitive magnetic layer is subjected to the influence of the magnetic field of the magnetic track, so that its magnetic orientation fluctuates. As a result, the magnetizations of the two magnetic layers go from an anti-parallel arrangement state to a parallel arrangement state, a variation of the resistivity of the two magnetic layers occurring at the transition between these two states.
It is possible to find detailed explanations on the magneto-resistance effect and the structure of magneto-resistance elements of the multilayer type in "Theory of giant magneto-resistance effects in magnetic layered structures with anti-ferromagnetic coupling," R. E. CAMLEY et al., Phys. Rev. Lett., V. 63 (6), page 664, 1989.
It should be noted that the magneto-resistance elements of the multilayer type have a greater structural complexity than the mono-layer type elements, but they offer the advantage of providing a more significant useful signal. Actually, the useful signal, i.e., the variation of the current which flows in the magnetic layers, is linked to the ratio of the variation of resistance .DELTA.3 R to resistance R between these two layers. This ratio is typically on the order of 10% in the case of the magneto-resistance elements of the multilayer type, while it is only on the order of 3% in the case of the monolayer type.